battery charger with pnp

Discussion in 'Power Electronics' started by hertz.12, Jul 20, 2017.

  1. hertz.12

    Thread Starter Member

    Apr 25, 2015
    40
    0
    hiii
    i found circuit that use pnp transistor and Comparator to charge battery..every thing work fine but i have one problem

    what will happens if i put the battery polarity reversed??
     
  2. noweare

    Member

    Jun 30, 2017
    64
    9
    You can put an Led and a resistor in parallel with the battery such that the Led is forward biased when the battery is put in backwards so that the light will come on. Size the resistor to set the LED current.
     
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  3. hertz.12

    Thread Starter Member

    Apr 25, 2015
    40
    0
    nice idea noweare
     
  4. #12

    Expert

    Nov 30, 2010
    18,078
    9,619
    BTDT. If you want to get fancy, You can use a dual color LED, red and green.
    I have done this.
    The limitation is the 4 volts. You can't run blue or white with a voltage that low.
     
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  5. hertz.12

    Thread Starter Member

    Apr 25, 2015
    40
    0
    thanks friend..i will try
     
  6. DickCappels

    Moderator

    Aug 21, 2008
    4,958
    1,508
    @hertz.12 Do you realize that the circuit you showed will try to change your battery to 5.0 volts? You might also want to add something to limit the current to protect your battery and the transistor.

    What is the circuitry to the right of the section you posted?
     
  7. hertz.12

    Thread Starter Member

    Apr 25, 2015
    40
    0
    hi..Comparator and diodes..transistor work as current limitting
     
  8. DickCappels

    Moderator

    Aug 21, 2008
    4,958
    1,508
    It looks like 8.6 ma of base current, more or less depending on the actual emitter voltage and temperature, and there is no visible mechanism for limiting the current other than the gain of the transistor. Unless I am misreading the chart that would limit the current to at least 6 amps.


    upload_2017-7-23_13-34-22.png
    Even if you want 6 amps, depending upon the gain of a transistor to limit current is not a reliable method. I suggest adding a resistor in series with the emitter as a minimum.
     
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